Juvenile and adult organisms may be more susceptible to lead neurotoxicity than commonly recognized. Recent data indicate that juvenile rats chronically exposed to lead in drinking water at concentrations as low as 50 ppm. (with corresponding blood lead values of 20-30 Mug/dl.) manifest distinctive behavioral phenomena. These appear as markedly elevated rates of responding in operant (schedule-controlled) behavior testing. Three issues require clarification: (1) Is the effect specific to rats and fixed-interval reinforcement schedules? (2) Is there a critical age period for its appearance? (3) What are the biological correlates, such as tissue lead concentrations and morphologic findings? These issues are addressed by a series of related experiments. Critical periods will be assessed by comparing juvenile rats (21 days old at the start of exposure) and adult rats (90 days old). Behavioral specificity will be explored in an experiment comparing two different reinforcement schedules, and species specificity assessed by studies in mice and guinea pigs. Finally, the biological correlates will be examined by extensive assays of tissue levels. The ultimate aim is a comprehensive evaluation of the previous findings and their possible biological mechanisms.